`TypeAliasType.__value__` might raise `NameError`, should we test it?

[sobolevn]

>>> type Invalid = dict[X, Y]

>>> Invalid[str, int]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Only generic type aliases are subscriptable

>>> Invalid.__value__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in Invalid
NameError: name 'X' is not defined

This seems correct to me.

However, this case is not covered right now with existing tests.

But, before adding a test case for this error (that in my opinion will be quite common among devs: when you forget to add [X, Y] to type alias definition), I want to double check this with @JelleZijlstra

Linked PRs

• gh-107905: Test raising __value__ for TypeAliasType #107997
• [3.12] gh-107905: Test raising __value__ for TypeAliasType (GH-107997) #108217

[JelleZijlstra]

Good catch, there is no test in test_type_aliases.py where evaluating the value raises.

[brandonardenwalli]

I am also try to follow this example, and I get a different error when I run example:

type Invalid = dict[X, Y] File "<stdin>", line 1 type Invalid = dict[X, Y] ^^^^^^^ SyntaxError: invalid syntax

And all code in >>> after does not work for me since this above error seems to be causing problem.

is this also a bug, or is there more I need to do? (I am doing something wrong?)

[JelleZijlstra]

@brandonardenwalli these issues refer to new syntax that is being added in Python 3.12. Please use a recent Python 3.12 build to test.

[brandonardenwalli]

@JelleZijlstra thank you for this very important information! Following the fix, I can now run the test cases and reproduce correctly:

>>> type Invalid = dict[X, Y]
>>> Invalid[str, int]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Only generic type aliases are subscriptable
>>> Invalid.__value__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in Invalid
NameError: name 'X' is not defined

Also, for anyone who may have trouble to find the 3.12. Go to https://www.python.org/download/pre-releases/ then pick a release. Right now, it looks to be https://www.python.org/downloads/release/python-3120rc1/ is the only option, so I use that one.